Mercedes W123 fuel sending unit disassembly and cleaning

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The fuel sending unit on your car, once removed, can be disassembled for cleaning and inspection before replacing it. This is especially helpful since 1) replacement parts are very expensive new from Mercedes and 2) used parts are just as likely to be faulty as the unit in your car currently. Use this guide to help take it down and maintain it.

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Turn the unit back over and lift the top out of the tube that makes up the body of the unit.

There are two small plates (one plastic and one metal) behind the nut you removed. Mind the order of these. The metal plate is on the outside and the plastic is on the inside. Also note the orientation of the plastic plate. There is an inside face, and outside face.

Inside you will find the float and the three wires that make the sending unit work. You can see the three wires pictured here.

Inspect these wires carefully. If any of them is broken they will need to be repaired by soldering them back on to their contact points.

If these wires are intact, proceed to clean the internal components (wires, float, etc) and the inside of the tube with brake cleaner.

Be very careful when handling the insides of the sending unit as the three wires are very fragile!

great guide but I was not careful and when I removed the nut on the bottom (gently with pliers) I was not holding on to the right part and dropped the inside unit breaking the copper colored wire. I was able to solder it back and got the unit back in the tank….so far, it seems the gauge is working…no jumping and appears to be going down as I drive. BE VERY CAREFUL with the unit, those wires are so thin and delicate. thanks for your site and all the advice!

Hi Kieron, I cannot say with certainty that this part is the same in the 409D. As you can see, this guide was done on, and written for, the W123 diesel. However, I can say with certainty that there are 3 wires on the W123 sending unit as pictured.

While cleaning I broke the silver wire that loops and counted as two wires. I installed a copper wire instead of silver wire. The unit/gauge indicated permanent full status. Apparently the resistance (ohm) difference between the copper and silver wires makes the unit work properly. All wires can be obtained by peeling off plastic covering from speaker cables and extracting a thin line of wire from it